The invention relates to electric lamps and particularly to automotive lamp capsules. More particularly the invention is concerned with blue tinted automotive lamp capsules.
With the advent of HID headlights, there has been a commercial demand for halogen lamps that provide bluer (less yellow) road lighting. The bluer light is believed to give better color perception, and may light up road marks and signs better. As a result there have been a variety of attempts to color tungsten halogen lamps without excessively reconstructing the existing lamp production lines. One method is to coat the exterior of the lamp with an interference coating. The interference layers filter the light, producing a bluer spectrum. The interference layers need to be coated accurately (about a quarter of a wave length), and numerous layers need to be built up to form an effective filter. The accurate industrial processing of such stack systems on a curved lamp capsule bulb is difficult to achieve, and only in rather slow or else inaccurate processes. None the less, lamps have been made, but the results have not been satisfactory. Interference layer coated lamp capsules are generally found to be non-compliant with existing legal standards. There is usually excessive glare as light is reflected or refracted from the various layers. Such lamp capsules tend to dazzle or glitter as a result. The interference layers also tend to cause color separation in the beam, so that as one views a beam from different angles one sees different colors. This makes viewing the illuminated roadway difficult, and is considered distracting to other drivers. The applicants have tested interference coated bulbs, and none have passed the glare requirements set forth by the SAE and the U.S. standard in F.M.V.S.S. section 108.
Alternatively, lamp capsules may be coated with colored resins. Although the filament produces an integrated color within the required SAE white region, the resin coated lamp capsules may still have a color separation. There are regions of non-compliant colored light emanating from the system particularly at the edges of the beam. Headlamp bulbs have relatively hot surfaces, running at temperatures up to 650 degrees Celsius. The existing resin dipped lamp capsules have generally been found to have limited life. The resin material on the hot lamp capsule glass, ages rapidly and fades, crazes, flakes, or peals off. The result is at best a lamp capsule that is no longer blue, and at worst a lamp capsule that has streaks or patches of blue and yellow light. Since the lamp capsule must produce additional lumens to account for the lumens that are filtered out, the lamp capsules are designed with higher initial lumen outputs. When the resin material flakes off, the lamp capsule can then be out of specification, emitting excessive light. In a similar fashion, light may be refracted or reflected from exposed portions of the failing resin coating, resulting in irregular lighting and glare.
Another alternative is to color the glass itself. Blue colored glass can be prepared using several different colorants. The colorant can be added to the glass composition. The precise color of the glass, or coating, depends on the oxidation state of the colorant and the matrix composition (i.e. ligand field effect). Weyl (Weyl, W. A., Coloured Glasses, Dawson""s of Pall Mall London, 1959), and Bamford (Bamford, C. R., Glass Science and Technology 2, Colour Generation and Control in Glass, Elsevier Scientific Publishing Co, NY 1977) reviewed the use of various compounds that contain chromium, cobalt, copper, iron, nickel, titanium and vanadium, to produce a blue color in glass. At this time, consumer demand for tinted headlamps is not so great that the manufacture of blue tubing stock is justified. Nor is the devotion of a complete lamp capsule production line to blue tinted lamp capsules justified.
In one alternative, Corning Inc. disclosed a method of converting clear tube stock to blue tinted stock (Blue High Silica Glass U.S. Pat. No. 5,610,107 issued Mar. 11, 1997). The Corning invention involves impregnating a porous high silica glass with a solution containing cobalt and other components, then re-consolidating the glass under oxidizing conditions to produce blue glass. While the Corning invention produces blue colored glass, the method of obtaining it is time consuming and requires shifting a lamp capsule production machine to run according to the new glass""s characteristics. It is also unlikely that the impregnation process could be used to color already finished lamp capsules.
There is then a need for method to color coat complex shaped glass objects, such as a finished headlamp capsule. The coating processing must be compatible with the existing capsule construction process. For example, any firing of the coating must be at or below the glass transition temperature of the glass substrate. There is also a need for a lamp capsule coating wherein the coating adheres or is bonded to the glass surface during normal operation of the lamp capsule. There is a need for a coating that can withstand elevated temperatures without degrading optically (fading or changing color) or physically (peeling, crazing, or flaking). There is a need for a coating that is sufficiently durable that it can mechanically withstand ordinary handling. There is a need for a coated lamp capsule in combination with existing headlamp reflectors that meets SAE and legal specifications for glare, color, total lumens, and light distribution.
A blue tinted automobile lamp capsule including a glass envelope; and a tungsten halogen light source enclosed in the envelope; may be durably coated with a blue tinted luster including gold, titanium oxide and bismuth oxide to form a thin surface skin on the exterior of the envelop. The resulting coated lamp provides a headlamp beam with improved night vision characteristics.